1. Field of the Invention
The present invention relates to an active matrix display device used as a display device for a computer display screen, a television, a light bulb of a projector or the like, and a method for correcting a defect thereof.
2. Description of the Related Art
A display device such as a liquid crystal display device or a plasma display device includes a plurality of pixel electrodes arranged in a matrix, and a plurality of counter electrodes disposed so as to oppose the respective pixel electrodes. A display medium such as liquid crystal or plasma is interposed between the pixel electrodes and the counter electrodes. In such a display device, when a driving signal is applied to the pixel electrodes, the display medium is optically modulated due to a voltage generated between the pixel electrodes and the counter electrodes. By selectively applying a driving signal to the pixel electrode taking advantage of the modulation principle, a display pattern is displayed on a screen.
One known method for driving such a display device is an active matrix driving method. An active matrix display device includes an active matrix substrate, a counter substrate disposed so as to oppose the active matrix substrate, and a display medium interposed therebetween. In the active matrix substrate, a plurality of pixel electrodes disposed in a matrix are respectively connected to switching elements so that an electric potential is selectively applied to the respective pixel electrodes via these switching elements. In general, a thin film transistor (TFT), a metal-insulator-metal (MIM) element or the like is used as the switching element.
In the conventional active matrix substrate described above, one of the bus lines, that is, a signal line, a scanning line or the like, is generally formed in the same layer as that of the pixel electrodes so as not to be in contact therewith. In recent years, an active matrix substrate having another configuration has been described (Japanese Laid-Open Patent Publication No. 61-156025). In such an active matrix substrate, an insulating film is provided so as to cover the signal lines or the scanning lines. A plurality of pixel electrodes are formed on the insulating film, and the pixel electrodes are connected to their respective switching elements. Since the pixel electrodes and the bus lines are separately formed in different layers in this configuration, it is possible to prevent an aperture ratio from being lowered by increasing the area of the pixel electrodes.
FIG. 15 is a plan view showing one pixel of the active matrix substrate having the configuration described in the above Patent Publication. In this active matrix substrate, scanning lines 52 and signal lines 53 are provided so as to cross each other. An insulating film (not shown) is formed so as to cover the scanning lines 52 and the signal lines 53. A pixel electrode 51 is formed on the insulating film. The pixel electrode 51 is connected to a drain electrode 52 of a thin film transistor (TFT) 55 via a contact hole 51b formed through the insulating film. By forming the pixel electrode 51 on the insulating film covering the scanning lines 52 and the signal lines 53 (the bus lines), the pixel electrode 51 and the bus lines 52 and 53 are separately formed in different layers.
The active matrix substrate shown in FIG. 15 has a Cs (storage capacity) on Common configuration. Specifically, a Cs line 59 common to the pixels is provided so as to be parallel to the scanning line 52. On the Cs line 59, a Cs electrode 56 is formed via a gate insulating film (not shown). The Cs electrode 56 is connected to the pixel electrode 51 via the contact hole 51a formed through the insulating film. A storage capacitor is constituted by the overlapping portion of the Cs line 59, the gate insulating film, and the Cs electrode 56.
In the active matrix display device using such an active matrix substrate, the disconnection of a bus line disadvantageously becomes a problem due to a defect occurring upon fabrication. In recent years, the width of the bus line has been minimized so as to prevent the aperture ratio from being lowered and thereby improving the accuracy of the display device. On the other hand, since the number of bus line intersections increases, a disconnection of a bus line and a leak at the intersection of the bus lines is more likely to occur as compared with a conventional active matrix display device. When a defect such as a disconnection of bus lines or a leak occurs, such a defect appears as a line defect on the display because a normal voltage is not applied from the bus line to the pixel electrode. The line defect is fatal for the display device; the display device which turns out to have a line defect is discarded as a defective product. As a result, the ratio of acceptable display device products is lowered thereby increasing the fabrication cost thereof.
In order to eliminate the above-mentioned problems resulting from the defects, i.e., the disconnection of the bus lines, an active matrix liquid crystal display device, in which two bus lines are provided for one pixel electrode, has been described (SID' 95 DIGEST of TECHNICAL PAPERS 4: AMLCDs 4.3; "High-Aperture and Fault-Tolerant Pixel Structures for TFT-LCDs"). FIG. 16 is a plan view showing an active matrix substrate of the liquid crystal display device.
In the active matrix substrate shown in FIG. 16, two scanning lines 52 and 52' are provided for each pixel electrode 51. Each of the scanning lines 52 and 52' is short-circuited by short-circuit lines 54 and 54' which are respectively provided along signal lines 53 and 53'. Each of the short-circuit lines 54 and 54' overlaps the pixel electrode 51 via an insulating film (not shown). The overlapping portion of the short-circuit lines 54 and 54' and the pixel electrode 51 serves as a storage capacitor.
A TFT 55 is driven by the two scanning lines 52 and 52' in the liquid crystal display device using such an active matrix substrate. Thus, even if a disconnection occurs in one of the two scanning lines 52 and 52', it is possible to apply a scan voltage to the TFT 55 via the short-circuit lines 54 and 54'. Moreover, since the short-circuit lines 54 and 54' are provided so as to partially overlap the pixel electrode 51 on the active matrix substrate side, part of a light-shielding pattern formed on the counter substrate so as to prevent light from leaking from a region between the adjacent pixel electrodes 51 can be omitted.
In order to reduce the above-mentioned problems resulting from the disconnection of bus lines, the Applicant of the present invention has described an active matrix substrate as shown in FIGS. 17 and 18 in Japanese Patent Application No. 7-251339, "Active matrix liquid crystal display device and Method for correcting pixel defect". FIG. 17 is a plan view showing the structure of an active matrix substrate, and FIG. 18 is a plan view specifically showing one pixel of the active matrix substrate shown in FIG. 17. In the illustrated active matrix substrate, a signal line 2 and a spare line 105 are alternately provided at predetermined intervals in the same layer so as to parallel to each other. The signal line 2 and the spare line 105 are connected to each other by a short-circuit line provided in the same layer. With this configuration, even if the disconnection of the signal line 2 occurs, the defective portion of the signal line 2 where the disconnection occurs is avoided through the spare line 105. Therefore, a signal voltage can be applied to part of the signal line 2 which is positioned ahead of the disconnected portion.
This Japanese Patent Application No. 7-251339 also proposes an active matrix substrate as shown in FIG. 19. In this active matrix substrate, a scanning line 1 and a spare line 122 are alternately provided at predetermined intervals in the same layer so as to parallel to each other. Then, the scanning line 1 and the spare line 122 are connected to each other by a short-circuit line formed in the same layer. With this configuration, even if a disconnection of the scanning line 1 occurs, the disconnected portion of the scanning line 1 is avoided through the spare line 122. Therefore, a signal voltage can be applied to the part of the signal lines 1 which is positioned ahead of the disconnected portion.
As described above, various configurations have been described so as to eliminate the problems generated by the disconnection of bus lines. However, the above configurations have the following problems.
For example, in the case where two scanning lines (or signal lines) are provided for one pixel electrode in the same layer in the active matrix substrate having a general configuration as shown in FIG. 16, it is necessary to place the signal lines (or scanning lines) so as not to be in contact with the pixel electrodes. Therefore, since the size of the pixel electrode cannot be increased, the aperture ratio of the display device cannot be maintained. Since it is necessary to reduce the distance between the scanning lines (or the signal lines) so as to maximize the aperture ratio under such a restriction, a leak is likely to occur between the two scanning lines (or signal lines).
Since the signal line and the spare line are formed in the same layer on the active matrix substrate shown in FIGS. 17 and 18, there is a possibility that a leak may occur not only at the intersection of the signal line and the scanning line but also at the intersection of the spare line and the scanning line. Accordingly, in the case where a leak between the signal line and the scanning line is detected while measuring the resistance between terminals of the respective lines, it is extremely difficult to specify the portion where the defect occurs: that is, it cannot be clearly distinguished whether the defect occurs at the intersection of the signal line and the scanning line or at the intersection of the spare line and the scanning line. Therefore, a special method is needed for determining the leak region to be corrected. Moreover, since the spare line traverses the pixel electrode with the insulating film interposed therebetween, an electric capacitor is generated between the pixel electrode and the spare line. In order to reduce this capacitance effect, it is necessary to use a material having a low dielectric as the insulating film material or to apply a special driving signal. Moreover, in order to prevent the aperture ratio of the display device from being lowered, it is necessary to reduce the line width of the spare line or to form the spare line using a transparent conductive film such as Indium Tin Oxide (ITO). Since the signal line and the spare line are formed in the same layer, the signal line and the spare line should be formed using the same material. However, in the case of a large display device, the length of the bus line is correspondingly increased. Thus, it is difficult to form the signal lines by using a material having a high specific resistivity such as ITO. Accordingly, a metal material having a low specific resistivity such as aluminum is used as the material of the spare line. Since the metal materials are opaque, the width of the spare line should be reduced so as not to reduce the aperture ratio. Thus, there is the possibility that disconnection of the spare line occurs.
Furthermore, since the scanning line and the spare line are formed in the same layer in the active matrix substrate shown in FIG. 19, the same problems are those of the active matrix substrate shown in FIGS. 17 and 18 described above also arise.